The present invention relates to a process for coating particles and to the coated particles obtained.
Particles of the core-shell type provide two benefits. On the one hand, they make it possible to increase the specific surface area of a material by dispersing it in the form of nanoparticles, thus causing a significant increase in its activity, or to isolate a particle from other particles by a protective layer and thus to modify the properties of the medium. On the other hand, in the case of the production of organic, mineral or hybrid composites, the coating of the particles makes it possible for the particles to be made compatible with the matrix. Mention may be made, for example, of the use of nanometric magnetic particles for recording data in the data processing field. Mention may also be made of the use of particles as solder binder in the electronics field. In the medical field, magnetic particles coated with organic substances are used.
Various processes for depositing a thin layer on a substrate are known. Particularly effective processes use a fluid raised to a pressure and to a temperature which are above the normal conditions, and especially a fluid placed under conditions very close to the critical pressure and critical temperature. These processes consist in depositing a film on a plane substrate, generally heated, placed in a reactor, by means of a supercritical fluid containing a precursor of the compound constituting the film, said precursor being converted before being deposited on the substrate, and the solvent for the fluid being removed by reducing the pressure in the reactor.
For example, xe2x80x9cOleg A. Louchev, et al., J. of Crystal Growth 155 (1995), 276-285xe2x80x9d describes a process consisting in depositing copper on a heated substrate consisting of a silicon grid placed in a reactor under high pressure, by means of a supercritical fluid containing copper hexafluoroacetylacetonate as copper precursor. Conversion of the precursor is obtained by heating to a temperature of around 600 to 800xc2x0 C.; this results in pyrolysis of the organic part of the precursor, which contaminates the substrate with carbon and with oxygen.
xe2x80x9cJ. F. Bocquet, et al., Surface and Coatings Technology, 70 (1994), 73-78xe2x80x9d describes a process for depositing a film of metal oxide (TiO2) on a heated substrate placed in a reactor, using a supercritical solution of a TiO2 precursor introduced into a pressurized reactor.
U.S. Pat. No. 5,789,027 (1996) describes a process for depositing a material on the surface of a substrate or within a porous solid. The process consists in dissolving a precursor of the material in a solvent under supercritical conditions, in bringing the substrate or the porous solid into contact with the supercritical solution, in adding a reactant which converts the precursor, thus causing the material to be deposited on the surface of the substrate or in the porous solid, and then in reducing the pressure in order to remove the solvent.
xe2x80x9cYa-Ping Sun, et al., Chemical Physics Letters 288 (1998), 585-588xe2x80x9d describes the preparation of CdS nanoparticles coated with a film of polyvinylpyrrolidone. A solution of Cd(NO3)2 in ammonia, brought under supercritical temperature and pressure conditions, is subjected to rapid expansion at room temperature in a solution of Na2S which also contains polyvinylpyrrolidone (PVP). The expansion causes precipitation of the Cd(NO3)2 and makes the Cd(NO3)2 react with the Na2S, thereby allowing CdS nanoparticles to form. Because the Na2S solution contains PVP, the CdS particles obtained are coated with PVP. This process makes it possible to prepare the particles in situ and at the same time to coat them. However, rapid expansion for the formation of particles to be coated is not very simple to implement as it involves passing a solution of particle precursors through a nozzle. A very small amount of material can be treated at each pass through the nozzle and the risks of blockage are not negligible. Furthermore, the rapid expansion is limited to particle precursors which may be dissolved in a supercritical solvent before the rapid expansion. Finally, the rapid expansion is obtained by a sudden drop in the pressure, which requires precise control of the nozzle temperature since the pressure reduction causes significant cooling.
It is an object of the present invention to provide a process allowing porous or nonporous particles to be simply and reliably coated with the aid of a precursor of the coating compound.
This is why the subject of the present invention is a process for depositing a film of a coating material on the surface of particles, or in the pores of porous particles, said process being characterized in that it consists in:
a) bringing, on the one hand, the particles to be coated and, on the other hand, an organometallic complex precursor of the coating material, optionally combined with one or more additional precursors which are organometallic complex or not, into contact in a fluid containing one or more solvents, said particles being kept dispersed in the fluid subjected to supercritical or slightly subcritical temperature and pressure conditions;
b) causing, within the fluid, the precursor of the coating material to be converted so that it is deposited on the particles;
c) bringing the fluid into temperature and pressure conditions such that the fluid is in the gaseous state in order to remove the solvent.
Within the context of the present invention, the term xe2x80x9cparticlexe2x80x9d is understood to mean any object which has a mean size of less than one millimeter, whatever its shape. The process of the present invention is particularly suitable for coating particles of very small size, and especially for nanometric particles and micrometric particles, in particular for particles having a mean size of between 1 nm and 100 xcexcm. The process is also very suited for coating particles having a complex shape. The particles may consist of a single chemical compound or by a mixture of compounds. The compounds may be mineral compounds, organic compounds or a mixture of organic or mineral compounds. The particles consisting of a mixture of compounds may be substantially homogeneous particles. However, they may also be heterogeneous particles in which the compound forming the core is different from the compound forming the external layer.
Within the context of the present invention, the fluid containing the particles to be coated and the precursor of the coating material is placed under supercritical or slightly subcritical temperature and pressure conditions. The term xe2x80x9csupercritical conditionsxe2x80x9d is understood to mean conditions under which the temperature is above the critical temperature Tc and the pressure is above the critical pressure Pc. The term xe2x80x9cslightly subcritical conditionsxe2x80x9d is understood to mean temperature T and pressure P conditions such that all the gases of the reaction mixture are dissolved in the liquid phase. The supercritical or slightly subcritical conditions are defined with respect to the pressure and to the temperature at the critical point Pc and Tc of the entire fluid constituting the reaction mixture. They generally lie within the range 0.5 less than Tc/T less than 2, 0.5 less than Pc/P less than 3. The reaction mixture consists of one or more solvents and various compounds in solution or in suspension. To a first approximation, the critical temperature and the critical pressure of such a fluid may be considered to be very close to those of the predominant solvent present in the fluid, and the supercritical or slightly subcritical conditions are defined with respect to the critical temperature and pressure of said predominant solvent. In general, the temperature of the fluid will be between 50xc2x0 C. and 600xc2x0 C., preferably between 100xc2x0 C. and 300xc2x0 C., and the pressure of the fluid will be between 0.2 MPa and 60 MPa, preferably between 0.5 MPa and 30 MPa. The particular values are chosen according to the precursor of the coating material.
The particles to be coated are kept dispersed in the reaction mixture by mechanical stirring, by natural convection or by forced convection, by the action of ultrasonics, by the creation of a magnetic field, by the creation of an electric field, or by a combination of several of these means. When the particles are kept dispersed by means of ultrasonics, it is preferred to use power ultrasonics, the frequency of which is from 20 kHz to 1 MHz. When the particles are kept dispersed by means of a magnetic field, a DC or AC magnetic field having an intensity of less than or equal to 2 tesla is imposed on the reaction mixture.
The reaction mixture essentially consists of one or more solvents in which the precursor of the coating material is dissolved and the particles are in suspension. As solvent, it is possible to use a compound which is either gaseous or liquid under standard temperature and pressure conditions, that is to say at 25xc2x0 C. and 0.1 MPa. For example, the solvent may be water or an organic solvent which is liquid under standard temperature and pressure conditions, or a mixture of such solvents. Among solvents which are liquid under standard temperature and pressure conditions, mention may be made of alkanes which have from 5 to 20 carbon atoms and which are liquid under standard temperature and pressure conditions, more particularly n-pentane, isopentane, hexane, heptane and octane; alkenes having from 5 to 20 carbon atoms; alkynes having from 4 to 20 carbon atoms; alcohols, more particular methanol and ethanol; ketones, in particular acetone; liquid ethers, esters, chlorinated hydrocarbons and fluorinated hydrocarbons; solvents resulting from petroleum cuts, such as white spirit, and mixtures thereof. Among solvents which are gaseous under standard temperature and pressure conditions, mention may be made of carbon dioxide, ammonia, helium, nitrogen, nitrous oxide, sulfur hexafluoride, gaseous alkanes having 1 to 5 carbon atoms, (such as methane, ethane, propane, n-butane, isobutane and neopentane), gaseous alkenes having from 2 to 4 carbon atoms (such as acetylene, propane and 1-butyne), gaseous dienes (such as propydiene), fluorinated hydrocarbons and mixtures thereof. The solvent itself may in certain cases constitute a precursor of the coating material.
The organometallic complex precursor of the coating material may be chosen from the acetylacetonates of various metals, which make it possible to obtain coatings of various types depending on the reaction conditions. In the strict absence of oxygen, a metallic coating is obtained. In the presence of an oxidizer, such as O2, H2O2 or NO2 for example, an oxide coating is obtained. In ammoniacal medium, a nitride coating is obtained. Copper acetylacetonate or copper hexafluoroacetylacetonate are advantageously used to obtain copper or copper oxide Cu2O coatings. As additional precursor, it is possible to combine with the organometallic complex precursor any compound capable of participating in the formation of the coating material. This may be a second compound of an organometallic complex, or a different compound which may or may not react with the organometallic complex compound. By way of example, mention may be made of the use of Cu(hfa)2 dissolved in ammonia, the ammonia solvent acting as reactant for the formation of copper nitride from the Cu(hfa)2 precursor. The process of the invention thus makes it possible to obtain particles whose core has a diameter between 1 nm and 1 xcexcm and consists of nickel, silica, iron oxide or an SmCo5 alloy, which are coated with copper, copper oxide or copper nitride.
The chemical conversion of the precursor or precursors present in the reaction mixture may be carried out either thermally or by means of a chemical reaction, depending on the nature and the reactivity of the precursor. When the reaction mixture contains several precursors of the coating material, the various precursors may be converted at the same time or in succession, depending on their nature and their reactivity. A solvent may constitute one precursor.
In one particular method of implementing the process of the invention, the following steps are carried out:
a fluid comprising at least one precursor of the coating material dissolved in a solvent S1 is prepared;
the fluid is subjected to supercritical or slightly subcritical temperature and pressure conditions;
said fluid is brought into contact with the particles to be coated, which are dispersed in a solvent S2, and pressure and temperature conditions suitable for causing the conversion of the precursor are imposed on the reaction mixture, the particles being kept dispersed;
the reaction mixture undergoes a pressure reduction in order to remove the solvents.
In another method of implementing the process of the invention, the following steps are carried out:
a fluid containing at least one precursor of the coating material dissolved in a solvent S1 is prepared;
the fluid is brought under supercritical or slightly subcritical temperature and pressure conditions;
said fluid is brought into contact with the particles to be coated, these being dispersed in a solvent S2, the particles being kept dispersed, one or more additives capable of reacting with the precursor or precursors of the coating material are added and then temperature and pressure conditions capable of causing the conversion of the precursor are imposed on the reaction mixture;
the reaction mixture undergoes a pressure reduction in order to remove the solvents.
In both methods of implementation described above, the solvents S1 and S2 may be identical or different. A third solvent may be introduced into the fluid in order to improve the operating conditions, especially in order to reduce the critical temperature and critical pressure of the fluid, in order to increase the solubility of the precursor or precursors, or to reduce the conversion temperature of the precursor or precursors. A variant of these methods of implementation consists in bringing the fluid containing the precursor into contact with the particles to be coated before the fluid is brought under supercritical or slightly subcritical conditions.
In a third method of implementing the process of the invention, the particles to be coated may be prepared in situ. The reaction fluid then contains one or more precursors of the particles and one or more precursors of the coating material. It is possible to use precursors which are converted by the action of heat, the precursors of the particles having a conversion temperature below that of the precursors of the coating materials. It is also possible to use precursors which are converted by a chemical reaction with an additional reactant, provided that the conversion of the precursor of the particles takes place first.
In this case, the following steps are carried out:
a fluid comprising at least one precursor of the particles to be coated, dissolved in a solvent S2, is prepared;
said fluid is brought under supercritical or slightly subcritical temperature and pressure conditions;
the particles are formed by modifying the precursor or precursors, either by an increase in the temperature or by the action of a suitable reactant, and the particles formed are kept dispersed;
a fluid comprising at least one precursor of the coating material, dissolved in a solvent S1 is prepared;
the fluid containing the particles to be coated is brought into contact with the fluid containing the precursor or precursors of the coating material under supercritical or slightly subcritical temperature and pressure conditions, to ensure that they are well dissolved, and then the reaction mixture is subjected to conditions suitable for causing the conversion of the precursor of the coating material;
next, the reaction mixture undergoes a pressure reduction in order to remove the solvents.
In this method of implementation, it is also possible to add one or more additional solvents to the various fluids so as to adjust the properties of the reaction mixture. Likewise, it is possible to use, where appropriate, the same solvent for the fluid containing the precursor of the particles and for the fluid containing the precursor of the coating material. .This method of implementation includes several variants. The precursor of the particles may be converted either by a heat treatment or by the addition of a suitable reactant. Likewise, the precursor of the coating material may be converted either by a heat treatment or by the addition of a suitable reactant. The fluids may be placed under supercritical or slightly subcritical conditions when they contain all their constituents or when they contain some of them. The condition common to all the variants is that the reaction mixture is under supercritical or slightly subcritical conditions at the moment when the precursor of the coating material is chemically converted.
The process of the invention may be implemented in order to deposit several coating layers on particles. For this purpose, all that is required is to introduce into the reaction mixture several precursors having a different reactivity and to impose on the reaction mixture, in succession, the conditions appropriate for causing the stepwise conversion of the precursors.
The process of the invention may be carried out continuously or in batch mode.